Search for: All records

Optical observations of transient luminous events and remote-sensing of the lower ionosphere with low-frequency radio waves have demonstrated that thunderstorms and lightning can have substantial impacts in the nighttime ionospheric D region. However, it remains a challenge to quantify such effects in the daytime lower ionosphere. The wealth of electron density data acquired over the years by the Arecibo Observatory incoherent scatter radar (ISR) with high vertical spatial resolution (300-m in the present study), combined with its tropical location in a region of high lightning activity, indicate a potentially transformative pathway to address this issue. Through a systematic survey, we show that daytime sudden electron density changes registered by Arecibo’s ISR during thunderstorm times are on average different than the ones happening during fair weather conditions (driven by other external factors). These changes typically correspond to electron density depletions in the D and E region. The survey also shows that these disturbances are different than the ones associated with solar flares, which tend to have longer duration and most often correspond to an increase in the local electron density content.

 

Multi‐resolution analysis methods can reveal the underlying physical dynamics of nonstationary signals, such as those from lightning. In this paper we demonstrate the application of two multi‐resolution analysis methods: Ensemble Empirical Mode Decomposition (EEMD) and Variational Mode Decomposition (VMD) in a comparative way in the analysis of electric field change waveforms from lightning. EEMD and VMD decompose signals into a set of Intrinsic Mode Functions (IMFs). The IMFs can be combined using distance and divergence metrics to obtain noise reduction or to obtain new waveforms that isolate the physical processes of interest while removing irrelevant components of the original signal. We apply the EEMD and VMD methods to the observations of three close Narrow Bipolar Events (NBEs) that were reported by Rison et al. (2016,https://doi.org/10.1038/ncomms10721). The ΔE observations reveal the occurrence of complex oscillatory processes after the main NBE sferic. We show that both EEMD and VMD are able to isolate the oscillations from the main NBE, with VMD being more effective of the two methods since it requires the least user supervision. The oscillations are found to begin at the end of the NBEs' downward fast positive breakdown, and appear to be produced by a half‐wavelength standing wave within a weakly‐conducting resonant ionization cavity left behind in the wake of the streamer‐based NBE event. Additional analysis shows that one of the NBEs was likely initiated by an energetic cosmic ray shower, and also corrects a misinterpretation in the literature that fast breakdown is an artifact of NBE‐like events in interferometer observations.

 

In the last couple of decades, substantial research has been dedicated to understanding the coupling between atmospheric regions. Research on transient luminous events (TLEs) appeared and quickly intensified with the promise of TLEs serving as an optical remote sensing tool of the mesosphere and lower ionosphere. However, to date it remains challenging to obtain quantitative estimates of electron density changes in the ionospheric D region due to underlying lightning and thunderstorms. Arecibo’s incoherent scatter radar (ISR) capabilities for measuring ionospheric electron density with high resolution (300-m spatial resolution in the present study), combined with its tropical location in a region of high lightning incidence rates, indicate a potentially transformative pathway to address this problem. Through a systematic survey, we show that sudden electron density changes registered by Arecibo’s ISR during thunderstorm times are on average different than the ones happening during fair weather conditions (driven by other external factors). Electron density changes happening coincidentally with lightning activity have typical amplitudes of 10–90% between 80–90 km altitude, and in a selected number of cases can be reasonably correlated to underlying lightning activity. 

In this paper we reconstruct Griffiths and Phelps' seminal model of streamer systems to test if it can reproduce the key observational features of fast positive breakdown. We first confirm that our implementation is accurate by reproducing the original results. The model describes how a system of positive streamers exhibits an initial exponential charge growth, as a function of position or time, which rapidly transitions into a quadratic steady state. The charge growth is accompanied by substantial electric field enhancement near the onset location, creating favorable conditions for lightning initiation. Due to the relatively low conductivity of streamers (effectively zero in this model), the electric field enhancement is created by the charge deposited in the first few meters of propagation, in the scale length where the charge growth transitions from exponential to quadratic. The quadratic growth of charge, combined with conical system expansion, makes the surface charge density of the moving front constant. The resulting electric field ahead of the streamer system remains nearly constant during its propagation, consistent with the observations of fast breakdown, which reveal a nearly constant propagation velocity, independently of discharge polarity. Minimal changes to the model allow for simulation of narrow bipolar events, reproducing very well their characteristic bipolar electric field change waveform. Despite its simplicity, the Griffiths and Phelps model provides valuable physical insights in the relationship between fast positive breakdown and lightning initiation.

 

Simultaneous data from two interferometers separated by 16 km and synchronized within 100 ns were collected for a thunderstorm near Langmuir Lab on October 23, 2018. Analysis via triangulation followed by a least squares fit to time of arrival across all six antennae produced a three‐dimensional interferometer (3DINTF) data set. Simultaneous Lightning Mapping Array data enabled an independent calculation of 3DINTF accuracy, yielding a median location uncertainty of 200 m. This is the most accurate verified result to date for a two‐station interferometer. The 3D data allowed profiling the velocity of multiple dart leaders and K leaders that followed the same channel. 3D velocities calculated from the in‐cloud initiation site to ground ranged from 3 × 10⁶to 20 × 10⁶ m/s. Average velocity generally increased with subsequent leaders, consistent with increased conditioning of the channel. Also, all leaders showed a factor of 2–3 decrease in velocity as they proceeded over 15 km of channel. We speculate that the velocity decrease is consistent with energy lost in the reionization of the channel at the leader tip. This paper includes an appendix providing details of the triangulation technique used.

 

This study reports on spectroscopy results from a high‐speed optical spectrograph of two naturally occurring lightning return strokes. The two strokes occurred near Melbourne, FL and were from two separate flashes that were about 10 min apart and had National Lightning Detection Network (NLDN) peak currents of −19 and −63 kA. The larger peak current stroke was from a dart leader and was the last stroke in a 5 return stroke flash, while the −19 kA stroke originated from a stepped leader and was the only stroke in that flash. From the flash spectra, the return stroke channel temperature was calculated using the neutral lines of 715.7 nm (OI) and 777.4 nm (OI). In addition to the use of the neutral emission lines, the use of novel instrumentation and image processing techniques allowed the temperature to be calculated for nearly the entire visible channel (several km) and for long durations (several hundred μs). This enables temperature estimates on an unprecedented spatial and temporal scale, which show that the vertical temperature profile is not uniform across the channel. The lower altitudes are significantly hotter than higher altitudes near the time of the return stroke, with temperature gradients along the channel as large as 12,000 K/km. The rate of cooling of the channel is also initially 3–4 times larger at lower altitudes in comparison with the segments at higher altitudes. The stroke with the larger peak current shows larger maximum temperatures, larger temperature gradients along the channel, and also cools quicker.